The present invention generally relates to ring trip detection circuits, and more particularly to a ring trip detection circuit which makes a ring trip detection using a small number of parts.
A subscriber circuit which is provided for each subscriber in a switching system normally has the so-called BORSCHT functions. BORSCHT is an abbreviation for Battery feed, Overvoltage protection, Ringing, Supervision, Codec, Hybrid, and Test. Out of the BORSCHT functions, the R (Ringing) function includes a ringer transmitting function for transmitting a ringing (or call) signal and a ring trip function for cutting off the ringing signal if the subscriber OFF-hooks the telephone receiver during the ringing.
In order to realize the ring trip function, it is necessary to detect whether or not the subscriber OFF-hooks the telephone receiver during the ringing. For this detection, a ring trip detection circuit is provided. The ring trip detection circuit detects a change in a current which flows through a transmission resistor. It is desirable from the economical point of view that such a ring trip detection circuit is made up of a minimum number of parts.
FIG. 1 shows an example of a general subscriber circuit. This subscriber circuit includes a protection test part 12 which is coupled to a telephone set 11 and has the functions of protecting and testing a subscriber line, a relay 13 for switching, a battery feed and supervision part 14 which has the functions of supplying power and monitoring the subscriber line, a two-wire/four-wire switching part 15 which has the function of making a two-wire/four-wire switching, a codec (coder/decoder) part 16 which is coupled to a network 17 and has the functions of coding and decoding signals, a ringing signal source 18, a transmission resistor 19 for transmitting a ringing signal, and a ring trip detection circuit 20. This ring trip detection circuit 20 includes a current detection part 21, a lowpass filter 22, and a judging part 23 which judges the ring trip.
As described above, the subscriber circuit has the BORSCHT functions, and out of the BORSCHT functions, the R function includes the ringer transmitting function for ringing the subscriber's bell by transmitting a ringing signal and the ring trip function for stopping the ringing if the subscriber OFF-hooks the telephone receiver in response to the ringing of the bell.
Out of the BORSCHT functions, the B and S functions are realized by the battery feed and supervision part 14, the 0 and T functions are realized by the protection test part 12, the R function is realized by the part including the ringing signal source 18, the transmission resistor 19 and the ring trip detection circuit 20, the C function is realized by the codec 16, and the H function is realized by the switching part 15.
In FIG. 1, the relay 13 is ON and connected to the ringing signal source 18 during the ringing. Hence, the ringing signal from the ringing signal source 18 is transmitted to the telephone set 11 via the transmission resistor 19. In this state, it is possible to measure a transmitting current with respect to the telephone set 11 by detecting a voltage across the two terminals of the transmission resistor 19. The transmitting current is made up of only an A.C. component which flows through a capacitor within the telephone set 11 in a state where the bell is ringing but the subscriber does not OFF-hook the telephone receiver. On the other hand, the transmitting current also includes a D.C. component which flows through a speaking circuit in a state where the subscriber has OFF-hooked the telephone receiver. Accordingly, in this latter case, the D.C. current from a -48 V power source of the switching system and an A.C. current from the ringing signal source 18 are superimposed and flow through the transmission resistor 19, and the A.C. current increases in this case.
The ring trip detection circuit 20 detects that the subscriber has OFF-hooked the telephone receiver by detecting the voltage change across the two terminals of the transmission resistor 19. The current detection part 21 detects the current value of the current flowing through the transmission resistor 19. The lowpass filter 22 decreases the A.C. component from the current detected in the current detection part 21. The judging part 23 judges whether or not the telephone receiver is OFF-hooked based on the magnitude of a sum signal which is made up of an increase of the D.C. component and an increase of the A.C. component. In FIG. 1, a supervision output signal SCN1 is output from the battery feed and supervision part 14, and an off-hook judging signal SCN2 is output from the judging part 23 of the ring trip detection circuit 20. A controller (not shown) turns OFF the relay 13 while the off-hook judging signal SCN2 is generated so as to disconnect the ringing signal source 18, and in this case, the ringing signal to the telephone set 11 is cut off.
FIG. 2 shows an example of the conventional ring trip detection circuit 20. In FIG. 2, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
In FIG. 2, the current detection part 21 is made up of a differential amplifier circuit using an operational amplifier. The current detection part 21 includes an operational amplifier 24, and resistors Rs and Rf which determine the amplitude of the differential amplifier circuit. The lowpass filter 22 includes a resistor Ra and a capacitor Ca. The judging part 23 includes a comparator 25 which compares an output voltage of the lowpass filter 22 with a threshold voltage Vth. The comparator 25 outputs the off-hook judging signal when the output voltage of the lowpass filter 22 exceeds the threshold voltage Vth.
It is desirable for safety reasons to provide the ring trip detection circuit 20 at a part on the side of the telephone set 11 of the ringing signal source 18, and the transmission resistor 19 is inserted at this part. Accordingly, the voltage across the two terminals of the transmission resistor 19 on the side of the ringing signal source 18 is the voltage of the ringing signal source 18 itself, that is, an A.C. voltage of 70 to 100 V which has a frequency of 16 to 20 Hz and is superimposed on the -48 V voltage from the -48 V power source of the switching system. But on the side of the telephone set 11, the corresponding voltage is reduced by a voltage drop at the transmission resistor 19. A voltage corresponding to this voltage drop is required in the current detection part 21, and the differential amplifier circuit is used to detect the voltage corresponding to this voltage drop.
FIG. 3 shows an example of the circuit construction of the conventional operational amplifier 24. The operational amplifier 24 includes transistors Q11 through Q23, constant current sources IC1 through IC4, diodes D1 through D3, a resistor R and a capacitor C1 which are connected as shown.
In FIG. 3, the transistors Q11 through Q14 form a differential amplifier, and emitters of the transistors Q13 and Q14 are connected to a current mirror circuit which is made up of the transistors Q15 and Q16 and form an active load. Hence, if an input +IN increases, for example, a current flows in a direction i from a collector of the transistor Q14.
The transistors Q17 through Q19 form three stages of amplifiers and amplify the change in the input so as to generate an output at a collector of the transistor Q19. As a result, an output is obtained via an emitter-follower circuit which is made up of the transistors Q21 and Q22.
As may be seen from FIG. 3, the operational amplifier 24 is made up of a large number of transistors. In addition, each constant current source is formed by several transistors. Hence, there is a problem in that the number of parts forming the operational amplifier 24 is large. In addition, because the operational amplifier 24 as a whole has an extremely high gain, a negative feedback is normally provided when in use, but the capacitor C1 for phase compensation is essential in order to prevent oscillation and ensure stable operation. However, this capacitor occupies a large area when the operational amplifier 24 is made in the form of a large scale integrated circuit (LSI), and there is a problem in that the provision of the capacitor C1 prevents the size of the LSI from being further reduced.
Therefore, although the current detection part 21 of the conventional ring trip detection circuit 20 detects the current by use of the operational amplifier 24, the number of parts becomes large if this ring trip detection circuit 20 is simply made in the form of an LSI. Furthermore, because the operational amplifier 24 requires the capacitor C1 for phase compensation, a large mounting area is occupied by the capacitor C1 and it is not only difficult to reduce the size of the LSI but also difficult to reduce the cost of the ring trip detection circuit 20.